`PairGrid` doesnt support adding Seaborn plots

Doing the following,

```
using Pandas
using Seaborn

df = Pandas.DataFrame(Dict("a" => [12,3,14,8,7,9], "b"=> [120,33,714,.8,887,99]))
g = Seaborn.PairGrid(df)
g.map_diag(Seaborn.kdeplot)
```

Leads to this error,
```
ERROR: PyError ($(Expr(:escape, :(ccall(#= /Users/traj/.julia/packages/PyCall/l64G5/src/pyfncall.jl:43 =# @pysym(:PyObject_Call), PyPtr, (PyPtr, PyPtr, PyPtr), o, pyargsptr, kw)))))
ValueError('callable is not supported by signature')
File "/Users/traj/Documents/Discover/venv/lib/python3.8/site-packages/seaborn/axisgrid.py", line 1351, in map_diag
if "hue" not in signature(func).parameters:
File "/usr/local/Cellar/python@3.8/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/inspect.py", line 3093, in signature
return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
File "/usr/local/Cellar/python@3.8/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/inspect.py", line 2842, in from_callable
return _signature_from_callable(obj, sigcls=cls,
File "/usr/local/Cellar/python@3.8/3.8.5/Frameworks/Python.framework/Versions/3.8/lib/python3.8/inspect.py", line 2405, in _signature_from_callable
raise ValueError('callable {!r} is not supported by signature'.format(obj))
```